Increasing demand for high octane gasoline blended with lower aliphatic alkyl ethers such as octane boosters and supplementary fuels has created a significant demand for isoalkylethers, especially the C.sub.5 to C.sub.7 methyl, ethyl and isopropyl-t-alkyl ethers, such as methyl t-butyl ether, ethyl t-butyl ether, t-amyl methyl ether and t-amyl ethyl ether. Consequently, there is an increasing demand for the corresponding isoolefin starting materials such as isobutene, isoamylenes and isohexenes.
To obtain isoolefins, it is desirable to convert an olefin or alkene such as normal butene, to a methyl branched alkene, for example isobutylene, by mechanisms such as structural isomerization. Such converted isoolefins then can be reacted further, such as by polymerization, etherification or oxidation, to form useful products. Normal olefins containing four carbon atoms (1-butene, trans-2-butene and cis-2-butene) and five carbon atoms (1-pentene, trans-2-pentene, and cis-2-pentene) are relatively inexpensive starting compounds. Conventionally, butenes and amylenes, including to a minor extent isobutylene and isoamylene, are obtained as a by-product from refinery and petrochemical processes such as catalytic and thermal cracking units. Butenes are also conveniently obtained from butadiene via selective hydrogenation.
Zeolite materials, both natural and synthetic, are known to have catalytic properties for many hydrocarbon processes. Zeolites typically are ordered porous crystalline aluminosilicates having a definite structure with cavities interconnected by channels. The cavities and channels throughout the crystalline material generally can be of such a size to allow selective separation of hydrocarbons. Such hydrocarbon separation by the crystalline aluminosilicates essentially depends on discrimination between molecular dimensions. Consequently, these materials in many instances are known in the art as "molecular sieves" and are used, in addition to catalytic properties, for certain selective adsorptive processes. Zeolite molecular sieves are discussed in great detail in D. W. Breck, Zeolite Molecular Sieves, Robert E. Krieger Publishing Company, Malabar, Florida (1984).
Generally, the term "zeolite" includes a wide variety of both natural and synthetic positive ion-containing crystalline aluminosilicate materials, including molecular sieves. They generally are characterized as crystalline aluminosilicates which comprise networks of SiO.sub.4 and AlO.sub.4 tetrahedra in which silicon and aluminum atoms are cross-linked in a three-dimensional framework by sharing of oxygen atoms. This framework structure contains cavities and channels or interconnected voids that are occupied by cations, such as sodium, potassium, ammonium, hydrogen, magnesium, calcium, and water molecules. The water may be removed reversibly, such as by heating, which leaves a crystalline host structure available for catalytic activity. The term "zeolite" in this specification is not limited to crystalline aluminosilicates. The term as used herein also includes silicoaluminophosphates (SAPO), metal integrated aluminophosphates (MeAPO and ELAPO), metal integrated silicoaluminophosphates (MeAPSO and ELAPSO). The MeAPO, MeAPSO, ELAPO, and ELAPSO families have additional elements included in their framework. For example, Me represents the elements Co, Fe, Mg, Mn, or Zn, and EL represents the elements Li, Be, Ga, Ge, As, or Ti. An alternative definition would be "zeolitic type molecular sieve" to encompass the materials useful for this invention.
Developments in the art have resulted in formation of many synthetic zeolitic crystalline materials. Crystalline aluminosilicates are the most prevalent and, as described in the patent literature and in the published journals, are designated by letters or other convenient symbols. Various zeolites which have been specifically named and described are, for example, Zeolite A (U.S. Pat. No. 2,882,243), Zeolite X (U.S. Pat. No. 2,882,244), Zeolite Y (U.S. Pat. No. 3,130,007), Zeolite ZSM-5 (U.S. Pat. No. 3,702,886), Zeolite ZSM-11 (U.S. Pat. No. 3,709,979), Zeolite ZSM-12 (U.S. Pat. No. 3,832,449), Zeolite ZSM-23 (U.S. Pat. No. 4,076,842), Zeolite ZSM-35 (U.S. Pat. Nos. 4,016,245 and 5,190,736), Zeolite ZSM-48 (U.S. Pat. No. 4,375,573), Zeolite NU-1 (U.S. Pat. No. 4,060,590) and others. Various ferrierite zeolites including the hydrogen form of ferrierite, are described in U.S. Pat. Nos. 3,933,974, 4,000,248 and 4,942,027 and patents cited therein. SAPO-type catalysts are described in U.S. Pat. No. 4,440,871. MeAPO type catalysts are described in U.S. Pat. Nos. 4,544,143 and 4,567,029; ELAPO catalysts are described in U.S. Pat. No. 4,500,651, and ELAPSO catalysts are described in European Patent Application 159,624.
Two general classes of catalysts have been disclosed as particularly useful for isomerizing a linear olefin to the corresponding methyl branched isoolefin. These include the porous, non-crystalline, refractory oxide-based catalysts and the zeolitic-based catalysts.
Illustrative of the porous, non-crystalline refractory oxide catalysts are those described in U.S. Pat. Nos. 4,434,315, 5,043,523, 3,531,542, 3,381,052, 3,444,096, 4,038,337, 3,663,453, British Patent No. 2,060,424 and in an article by V. R. Choudhary and L. K. Doraiswamy, "Isomerization of n-Butene to Isobutene, I. Selection of Catalyst by Group Screening," Journal of Catalysis, volume 23, pages 54-60, 1971. All of these catalysts deactivate rapidly. According to the examples in British Patent No. 2,060,424, run life can be as short as 1 to 2 hours. Often, it is necessary to add steam and halogen compounds to prolong the catalyst run life. German specification No. 3,000,650-A states that the run life can be increased to approximately 50 hours by these methods although this is still less than desirable.
With regard to the zeolitic-based catalysts, the most significant use has involved large pore zeolites or zeolites having two or more-dimensional interconnecting channels. Illustrative of these materials are U.S. Pat. Nos. 4,503,282, 5,227,569, 4,435,311, and 4,392,003.
More recently, European Patent Publication Number 523,838 A2, published Jan. 20, 1993, has disclosed a process for structurally isomerizing a linear olefin to its corresponding methyl branched isoolefin using as a catalyst a zeolite with one or more one-dimensional pore structure having a pore size small enough to retard by-product dimerization and coke formation within the pore structure and large enough to permit entry of the linear olefin and allow formation of the methyl branched isoolefin (i.e. medium or intermediate pore zeolites). These catalysts are formed by blending a finely divided crystalline zeolite with a binder material and mulling the blended mixture by adding water and acetic acid. The resulting mixtures are then shaped, dried and calcined to form the catalyst composition.
However, it is desirable to have a more active and stable catalyst composition to obtain increased efficiency or overall yield of the desired isoolefins. Such an increase can be obtained by increase in run length, higher selectivity and/or higher activity of the catalyst used in the olefin isomerization process.
It is therefore an object of the present invention to provide a medium pore zeolite catalyzed process for structurally isomerizing a linear olefin to its corresponding methyl branched isoolefin with improved stability, efficiency and/or yield. It is another object of the present invention to provide a more stable catalyst composition useful in structurally isomerizing a linear olefin to isoolefins.